In general, lithium secondary batteries use a non-aqueous electrolyte due to the reactivity of lithium with moisture. The non-aqueous electrolyte may be a solid polymer containing a lithium salt or a liquid phase with a dissociated lithium salt in an organic solvent. The lithium secondary battery may be classified as a lithium metal battery, a lithium ion battery using a liquid electrolyte, or a lithium ion polymer battery using a solid polymer electrolyte.
There is no problem of organic electrolyte leakage in a pure solid type lithium ion polymer battery, and a gel type lithium ion polymer battery can also prevent the problem of electrolyte leakage in a simpler manner than the lithium ion battery using liquid electrolyte. For example, the lithium ion polymer battery can use a pouch instead of a metal can as a receptacle of an electrode assembly.
In general, the pouch may comprise a multilayer film including a metal foil layer and a plastic layer which covers the metal foil layer. Using such a pouch makes it possible to reduce the weight of a battery as compared with using a metal can. Usually, aluminum is used as metal constituting a foil in the multilayer film pouch. A polymer layer forming an inner layer of the pouch film protects the metal foil against an electrolyte and simultaneously prevents a short circuit among a positive electrode, a negative electrode and electrode tabs.
In order to manufacture a pouch type lithium secondary battery, an electrode assembly, which is formed by laminating or laminating and then winding a positive electrode, separator and negative electrode, is placed into a pouch in a provisionally sealed state. Subsequently, an upper pouch film and a lower pouch film are thermally fused to each other in open edge portions of the pouch to form a bare cell battery of a sealed pouch type.
Accessories or structures such as a protecting circuit module (PCM) or a positive temperature coefficient (PTC), not shown in the Figures described below, are attached to the bare cell battery to form a core pack battery. By coupling this core pack battery within a hard case, a complete hard pack battery is produced. The hard case can be formed by using polypropylene resin and so forth without separately providing a circuit or an electrical conductor portion on its inner side, but the inside of the hard case may be provided with a separate accessory circuit or other electrical conductor portion dependent on a device for which the battery is used. In some cases, the core pack battery may be used in such a manner that it is directly attached to a product without a separate hard case.
Referring to FIGS. 1 and 2, flange-shaped lateral edges 23 which have been fused at the sealing of a pouch are shown in a state that they are folded toward the front, that is, in a direction forming a groove on both left and right sides of a core pack battery 100 having an approximately rectangular shape. Folding the lateral edges 23 reduces the battery in size by as much as the widths of the lateral edges 23 in order to remove unused space in the bare cell battery which is formed by fusing the edges to seal the pouch.
A protecting circuit module and other structure are connected to the battery around an upper side, from which electrode tabs 37, 38 are drawn out, of four sides which forms a rectangle when viewed from the front of the core pack battery 100. Also, the electrode tabs 37, 38 are bent once or twice as shown in FIG. 2. As the electrode tabs are bent, an upper side edge 23′ from which the electrode tabs are drawn out in the sealed pouch, the protecting circuit module 51 is located in an empty space defined by one wall surface 56 forming the groove 54 on the side from which the electrode tabs are drawn out, and both the left and right edges of the pouch folded toward the groove, so that the length of the whole battery can be reduced.
Even if both the lateral edges 23 of the pouch are in a folded state, the metal foil constituting an intermediate layer of the pouch film is still exposed outside in end portions of the edges 23. Since an electrical conductor portion of the protecting circuit module 51 is positioned spatially close to the folded edges 23 of the pouch when the core pack battery 100 has been formed, it is highly likely to be electrically connected to the edges 23. If the electrical conductor portion of the protecting circuit module 51 is connected to the negative electrode of the battery in any manner, a possibility of a short circuit between the copper of the negative electrode and the aluminum foil is increased.
In addition, a short circuit between the metal foil of the pouch film and the negative electrode of the battery may occur by way of a circuit portion or other electrical conductor within the hard case or a battery box of a product for which the battery is used when the core pack battery is directly inserted into the hard case or the battery box. Otherwise, electrical connection may be established by a path passing through the metal foil of the pouch film, the electrical conductor of the protecting circuit module, the electrical conductor of the hard case or the battery box, and the negative electrode of the battery.
Once aluminum constituting the metal foil of the pouch film is connected to a copper collector of the negative electrode, the aluminum foil may be subject to corrosion. In particular, the corrosion 10 of the metal foil may be accelerated under an environment where leaked electrolyte component or moisture exists around the negative electrode tab in the pouch.
If the metal foil serving as a barrier to moisture or oxygen continues to corrode, the polymer layer of the pouch film may prevent entrance of moisture and oxygen. The lowering of blocking capacity of the pouch may give rise to abnormality of the battery. That is, if an organic electrolyte of a gel type electrolyte separator evaporates or foreign moisture or oxygen intrudes into the battery, abnormal phenomena such as swelling occur, which results in the lifetime of the battery being discarded, deteriorated or shortened.
In order to overcome these problems a method where the lateral edges are twice folded into the form of a flange by folding on both left and right sides of a core pack battery has been proposed. First, both the lateral edges 23 of the pouch are folded into halves to form overlapped edge portions 23, as designated by FIG. 3. As a result, the width of the edges is reduced by half and ends 231 of the edges come into contact with portions of sidewall surfaces 541 constituting the groove 54. Subsequently, the overlapped edge portions are folded again toward the groove 54 resulting in the ends 231 becoming invisible to the exterior while being interposed between the edges 23 and the sidewall surfaces 541 constituting the groove 54.
However, the protecting circuit module (not shown), which is connected to the electrode tabs 37, 38 when they are bent, is located in a region which is an empty space not occupied by the groove 54 of the pouch. Consequently, even if the edges 23 are folded twice, the ends 231 of the edges are not shielded by the sidewall surfaces 541 of the groove 54. In other words, the ends 231 of the edges are still exposed in that space, so that they are highly likely to be connected to the protecting circuit module or 10 the like, located in that space or elsewhere.